This is the 2474th most frequent Catalan word.
soterrani
"Basement" or "underground floor."
The word 'soterrani' is used here as a noun, denoting a basement, an underground level of a house or building.
El soterrani de casa està ple de trastos vells.
The basement of the house is full of old stuff.
Here, 'soterrani' again is a noun, referring to the location where the treasure was buried - an underground area.
Van soterrar un tresor al soterrani abans de marxar.
They buried a treasure in the basement before leaving.
In this sentence, 'soterrani,' used as a noun, refers to the underground part of the building where the refuge is constructed.